The present invention relates to a procedure and to an apparatus use in dry forming of a fibre layer.
The procedure and apparatus of the invention, which are hereinafter referred to by the common designation ‘solution of the invention’, are applicable for use for dry formation of a fibre layer in web material, e.g. in conjunction with paper manufacture. In this type of processes, a fibrous raw material, such as wood pulp, is generally defibrillated before the formation of a fibre layer e.g. in a hammer mill to produce separate fibres, which are passed with a conveying air flow via various devices associated with the process into a distributor unit called a former. The production line may contain several formers like this placed one after the other e.g. to produce several fibre layers. A typical former is e.g. a drum former, which may have one or more cylindrical drums or screen tubes side by side, with apertures or through holes in their cylindrical surface for spreading the fibre flow onto a formation surface on a moving forming wire below the former.
Placed inside the former drum, close to the inner surface of the drum, is generally a consecutive series of bladed wheels or a single elongated bladed wheel, which breaks up any lumps in the fibre flow and distributes the fibre flow as evenly as possible to the through holes in the drum. In a dual-drum former, the fibre flow is conveyed by a conveying air flow blown into the drums to produce a fibre layer as even as possible e.g. from the ends of the drums so that the fibre flow enters the drums from their opposite ends.
Below the former is a moving wire called forming wire, whose upper surface acts as a fibre layer forming surface. Below the wire, aligned with the former is a suction box, which produces a suction that draws fibres from the former to the forming surface.
The function of the former is to distribute the fibres as a layer as even as possible on the forming surface of the wire in order that that the product to be manufactured should be as homogeneous as possible. However, a problem with prior-art dry formation solutions is that the fibre flow cannot be distributed so as to form a sufficiently even layer over the entire width of the forming surface.
To solve the above-mentioned problem, improved versions of the above solutions have been developed. In one such a solution, the fibre flow is spread onto a forming surface by means of a downwards widening discharge conduit. Similarly, a supply conduit is divided into four separate discharge conduits. As the device has no flow adjustment in the supply or discharge conduit, the device requires the use of complex mixers to mix the fibre mass to produce a layer as even as possible on the forming surface of the wire. The drawback is a complicated and failure-prone construction that requires frequent maintenance.
U.S. Pat. No. 4,761,858 also discloses an improved structure for producing from the fibre flow a layer as even as possible on the forming wire. The apparatus is provided with air circulation and an intermediate chamber placed above the drum former and having in its upper and lower parts mutually perpendicular fins for adjustment of the passage of the fibre flow. The adjustment enables a more even formation of the fibre layer, but the problem here is that the adjustment can not be made during operation. Therefore it is not possible to achieve an ideal adjustment and the time required for the adjustment reduces the operating time of the apparatus. Moreover, the adjustment made from the fins is a complicated and difficult operation. An additional disadvantage is that the lattices formed by the fins are tight and are easily blocked, necessitating an interruption of production and cleaning of the lattices.